magnitude

A measure of the brightness of a celestial object, first used (120 BC) by Hipparchus, who referred to the brightest stars visible to the eye as ‘first magnitude’ and the dimmest as ‘sixth magnitude’. The system was given a scientific basis in 1856: equal magnitude steps are in logarithmic progression, such that a magnitude difference of one unit corresponds to a brightness ratio of 2·512, and five magnitudes correspond to a ratio of 100 in actual brightnesses. The zero of the magnitude scale is essentially arbitrary. The apparent magnitude of a star is its brightness measured at the Earth, which depends on distance and luminosity. More useful physically is absolute magnitude; this is the observed apparent magnitude converted to what the object would have at an (arbitrary) distance of 10 parsec. Properties of stars and galaxies can be properly compared only via absolute magnitudes.

Magnitude may refer to:

Magnitude (mathematics), a measure of the size of a mathematical object

In astronomy:

A logarithmic measure of the brightness of an astronomical object: Apparent magnitude Absolute magnitude Magnitude of an eclipse

In seismology:

Either of two measures of earthquake strength: Richter magnitude scale (including the surface-wave Ms and body wave Mb magnitudes) Moment magnitude scale

Other:

Magnitude is an attack in the Pokémon fictional world